Cottongrass Sedge-miner Elachista albidella

One of several species that were previously placed in the genus Biselachista. Formerly given the vernacular name Cotton-grass Dwarf.

Based on its external appearance I initially thought, correctly as it turned out, that this moth was very probably Elachista albidella, a species I had not recorded here before. I sometimes catch Bog Sedge-miner Elachista utonella here but this specis is usually much darker. But there are similarities and after a bit of research I realised that some utonella are much paler than any I've seen before - perhaps this could be a pale utonella? There are other (less likely) species formerly in the genus Biselachista that need to be carefully eliminated too (perhaps especially Elachista eleocharialla which has not been recorded in Norfolk), and so it was necessary to examine its genitalia.

It was a female and the position of the ostium ruled out eleochariella. Elachista utonella is probably the hardest alternative to discount from the genitalia - both have a spiculated colliculum (I think that's what it is) at the top of the ductus bursae, shaped like a filled-in U and with a concave top, and both have an area of lighter spiculation above this. The shape and side borders of this patch differ between the species so in albidella it is oblong-oval shaped and has clearly bordered sides whereas in utonella it lacks the clear sides and stretches up in the middle more.

The diagrams in MoGBI suggest that the sigma in the corpus bursae differ little between closely related species but seem to suggest difference in the shape of the lightly-sclerotised patch surorundingt the sigma - however these differences don't appear to be borne out by photos of those species at the Moth Dissection website. Note that in my photos the first photo showing the sigma before I'd set it looks right (if not very sharp); when I applied the cover slip to the slide the two sides to the cornuti separated away from each other and the sclerotised triangles also parted from one another.

female Cottongrass Sedge-miner Elachista albidella showing genitalia (ovipositor and signa before setting then various views after setting), North Elmham (Norfolk, UK), 24th June 2023

I had only seen this species once before and that individual wasn't dissected to confirm. The species is now grade 4 (requiring dissection to confirm).

Cottongrass Sedge-miner Elachista albidella, Upton Fen (Norfolk, UK), 26th July 2014